Device and method for remelting metallic surfaces

ABSTRACT

A method for remelting metallic surfaces of components using the effect of a stable high pressure plasma jet, includes melting the surface in localized areas, the surface having a structure refinement after solidification. The plasma jet action is generated by the microwave impact on a carrier gas, the pressure of the high pressure plasma jet being above the atmospheric pressure. In addition, a plasma torch for generating a directed high pressure plasma jet includes a gas supply, a device for generating a plasma, and an outlet nozzle for a plasma jet. The device for generating the plasma includes a magnetron and a resonator in which the supplied pressurized carrier gas is transferred into a plasma under the effect of microwaves, causing the plasma to exit through the outlet nozzle at a pressure above 0.1 MPa.

Priority is claimed to German Patent Application No. DE 10 2004 026 636.0, filed on Jun. 1, 2005, the entire disclosure of which is incorporated by reference herein.

The present invention relates to a method for remelting metallic surfaces using the effect of a high pressure plasma jet, the surface being melted in localized areas and having a structure refinement after solidification, the plasma jet action being generated by the microwave impact on a carrier gas and the pressure of the high pressure plasma jet being above the atmospheric pressure.

BACKGROUND

For metallic materials, alloy remelting represents a known method for enhancing the surface hardness, the surface strength, or the surface ductility. The change in the material properties is based on structure transformation which is caused by melting and quenching processes. The quick solidification of the melted surface layer is accompanied by structure transformation, a grain refinement for example, or the formation of metastable phases. It is frequently only necessary to treat the surface layer in localized areas of the material and to leave the base material outside these function surfaces unchanged.

It is known from CH 664 579 A5 to use a plasma welding apparatus in a method for remelting metallic surfaces using a plasma jet.

Different high-performance jet methods, such as laser remelting, are known for treating the surface layer of a work piece. The laser remelting method is associated with high investment and operating costs.

Plasma jet methods are also known as additional methods. The plasma jet methods are typically high-performance micro jet methods having the disadvantage of low jet quality and low power density. It is also disadvantageous that the choice of the type of plasma gas is very limited. Gases or gas mixtures of Ar, H₂, and N₂ are the only common choices. Due to the system inertia, regulation of the gas supply and the power may only take place in a delayed manner—the delay in plasma jet methods is several seconds at best.

In the laser remelting method as well as in the plasma jet method, a local area of the work piece surface, only a few μm to mm thick, is melted.

However, such alloy remelting methods do not conform to the increasing demands placed on the mass production of components with larger dimensions. In the automotive industry in particular, alloy remelting methods, directed in particular to increasing the strength and ductility of work pieces or components which are subjected to thermal-mechanical stress (TMF thermal-mechanical fatigue), are becoming increasingly important in order to replace the expensive coating methods. This is true, for example, for valve bars and/or valve seats of a light metal cylinder head.

A method for manufacturing a cylinder head for an internal combustion engine using an Al casting alloy is known from DE 3605519 A1, for example, in which, by directing energy of high power density such as a tungsten inert gas arc or laser energy, the surface of the aluminum alloy is melted and quickly cooled down again to solidify. Laser energy, plasma arcs, and electron beams are cited as further energy sources.

The described methods have the disadvantage that the energy sources used allow only low power densities, i.e., achieving high power densities involves substantial complexity with regard to equipment. This represents a disadvantage for the mass production of components to be remelted since this is associated in particular with long processing times and high processing costs. In addition, the focusing and stabilizing of conventional plasma jets are problematic at high plasma energy densities. Current plasma jet methods are pure surface methods and are not suitable for the remelting of deeper-lying areas of the surface.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for remelting metallic surfaces of components having energy densities on the metal surface in order to make shorter processing times possible in particular. A further or alternate object of the present invention is to provide a suitable plasma torch.

The present invention provides a method for remelting metallic surfaces of components using the effect of a stable high pressure plasma jet, the surface being melted in localized areas and having a structure refinement after solidification, wherein the plasma jet (9) is generated by the microwave impact on a carrier gas, the pressure of the high pressure plasma jet being above the atmospheric pressure. In addition, the present invention provides a plasma torch for generating a directed high pressure plasma jet including a gas supply, a device for generating a plasma, and an outlet nozzle for a plasma jet, wherein the device for generating the plasma includes a magnetron (13) and a resonator (5) in which the supplied pressurized carrier gas is transferred into a plasma under the effect of microwaves, causing the plasma to exit through the outlet nozzle (3) at a pressure above 0.1 MPa.

The method according to the present invention provides that, for remelting metallic surfaces, a stable high pressure plasma jet is directed over the surface of the component, the surface being melted in localized areas due to the impact of this stable high pressure plasma jet and having a structure refinement after solidification. The plasma jet action is generated by the microwave impact on a carrier gas, the pressure of the high pressure plasma jet being above the atmospheric pressure.

This method has the advantage that a high energy density, i.e., power density of the jet, may be achieved on the component's surface.

The high power density depends in particular on the high pressure, i.e. the high density of the plasma gas. Due to the density, the number of energy-transferring gas atoms or molecules per volume unit is increased. According to the present invention, the pressure of the plasma jet, at least at the outlet aperture, is above 0.1 MPa, preferably in the 0.1 MPa to 0.8 MPa range, particularly preferably in a 0.15 MPa to 0.4 MPa range. Too high a pressure is difficult to achieve by the equipment. And also, too high a pressure of the plasma jet results in an undesirable blowing of the melted surface.

A further advantageous effect of the high pressure method is the fact that the microwave source enters the carrier gas with a high thermal degree of efficiency. The pressure in the microwave device, generated by a microwave resonator in particular, is preferably in the 0.1 MPa to 0.8 MPa range.

Another advantage of the present invention is the fact that the selection of carrier gases, which form the plasma jet, is hardly limited. When choosing, a distinction should be made between inert gases and reactive gases in particular, which, depending on the application, may also be used combined in a suitable manner. The carrier gases preferred according to the present invention are the gases Ar, He, N₂, H₂, O₂, CO₂, H₂O, CH₄, and/or C₂H₆, which may be found in pure form or in different gas mixtures with each other.

Air is used as the carrier gas in a preferred embodiment of the present invention.

If only a pure remelting process is intended, then inert gases, Ar in particular, are preferred carrier gases.

If reactive gases, such as O₂, N₂, or H₂O, are used, a partial reaction of the superficial light metal with the reactive gas takes place. This causes light metal oxides in particular, or nitrides, e.g., Al₂O₃ or AlN, to be formed. These ceramic reaction products are incorporated into the remelted surface layer, causing an advantageous dispersion gain.

A further advantage of the method according to the present invention is the fact that a comparatively stable plasma jet is used which is able to be accurately directed onto the surface of the component to be treated. Furthermore, it is possible to geometrically modify the gas dynamics of the plasma jet, i.e., to fan out, for example, or to focus it.

In a preferred variant, a filament-shaped plasma jet having a length above 5 cm is used. In a particularly preferred variant, the plasma jet has a diameter in the 0.5 cm to 5 cm range and a length in the 10 cm to 40 cm range.

By varying the plasma jet diameter and the speed with which the plasma jet is guided over the surface, different remelting temperatures and/or different cooling temperatures of the melt may be achieved for a given output of the plasma torch.

The method according to the present invention for remelting metallic surfaces is advantageous in particular when components made of light metal alloys are used. This includes the current aluminum alloys.

The components which are able to be handled particularly advantageously using the method according to the present invention include cylinder heads in particular.

If the usual aluminum alloys are used, the energy, entered via the plasma jet, is preferably set in such a way that a cooling rate in the 20 K/sec to 110 K/sec range is achieved.

In cylinder heads made of aluminum alloy, the remelting is preferably set in such a way that a structure in the T7 state is formed. The remaining cylinder head typically has a T6 structure.

The remelted surface layer preferably has a thickness, i.e. depth, in the range of a few 100 μm to a few mm. A thickness is preferably set which is in the 0.5 mm to 1.5 mm range. However, due to the method according to the present invention using a high pressure plasma jet, it is also possible to set considerably thicker layers, in the range of several mm for example, without great additional costs. This may be an advantage if, for example after remelting, targeted areas of the surface should be remachined to remove chips without the remelted material layer becoming completely lost.

According to the present invention, the plasma jet has a comparatively high power density in order to be able to implement short processing times.

The surface is preferably treated using a plasma jet having a power density in the 6 kW/cm² to 20 kW/cm² range, the jet being moved over the surface at a speed of 2 mm/sec to 4 mm/sec.

In a further preferred variant, the plasma jet has a power density in the 20 kW/cm² to 60 kW/cm² range and is moved over the surface at a speed of 2 mm/sec to 10 mm/sec.

In a further advantageous embodiment of the present invention, solid or liquid substances are supplied to the plasma jet close to the nozzle outlet aperture. This may take place either prior or subsequent to opening the nozzle. In terms of design, it must be borne in mind that remixing of the supplied substances into the gas chamber of the resonator is largely impossible.

In a first variant of this embodiment, the solid substances are formed by ceramic powders. These particles preferably have a nano structure and have particle sizes essentially below approximately 1μ, in particular below approximately 500 nm. The ceramic particles are inserted into the melted surface layer via the plasma jet and are dispersed in the melt layer, causing a dispersion gain of the metal layer. An increase in particular in vibrostability under thermo mechanical stress of the local surface area is achieved via the nano-structured particles. The increase in vibrostability is based on the dispersion gain of the local area of the surface layer due to the finely distributed nano-structured particles as well as on the dependency of the yield point from the grain refinement (Hall-Petch relationship) which is brought about by the remelting process.

The preferred ceramic particles include oxides such as Al₂O₃, or nitrides such as AlN, Si₃N₄ and/or carbides such as SiC.

The solid or liquid substances are preferably supplied to the plasma jet via a ring nozzle. The use of a ring nozzle results in a homogenization of the nano-structured particles in the plasma jet as well as in the melted surface layer of the metallic work piece.

The liquid substances may be supplied in an analogous manner. The preferred liquid substances include solutions of metal salts, e.g., metal hydroxides or metal carboxylic acid salts, or solutions of metal-organic compounds, e.g., silanes, carbosilanes, or metal chelate compounds. The liquid substances decompose under the plasma jet's conditions into the corresponding metal oxides, metal nitrides, or metal carbides. These act in an analogous manner as the supplied ceramic particles. The particles supplied via the decomposition of the liquid substances are generally distinctly finer than those obtainable via the supply of the solid substances.

Another aspect relates to the application of the remelting method via high pressure plasma jet on components made of light metal alloy. A preferred application is the remelting of surface layers of cylinder heads, preferably in the valve bar and/or valve seat area(s).

Another aspect of the present invention relates to a device for generating a high pressure plasma jet, hereinafter referred to as a plasma torch, using microwave energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The configuration of the method and the plasma torch according to the present invention is subsequently explained in greater detail with reference to the drawings, in which:

FIG. 1 shows a schematic representation of a plasma torch including short-circuiting plunger (1), aperture (2), nozzle (3), observation window (4), resonator (5), inspection glass (6), gas feeder (7), glass holder (8), plasma jet (9), water load (10), circulator (11), frequency tuner (12), and magnetron (13); and

FIG. 2 shows the remelting process including melted surface layer (11), metallic surface of a component (21), supply device (31), supplied particles (41), and plasma jet (9).

DETAILED DESCRIPTION

The carrier gas is supplied to the plasma torch via a gas feeder (7) for generating a directed high pressure plasma jet. At least during the feeding of microwave energy, the gas is pressurized with an overpressure. A pressure above 0.1 MPa is preferably set, particularly preferred in the 0.2 MPa to 0.8 MPa range. The microwave energy is generated in a magnetron (13) and acts on the carrier gas in resonator (5). Common frequencies are around 0.95 GHz to 12 GHz. A frequency of 2.45 GHz is particularly preferred. The output of the magnetron depends in particular on the intended power density of the plasma jet. Typical values are in the 1 kW to 20 kW range.

The microwaves are conducted to resonator (5) via a hollow conductor system and generate the plasma by resonant coupling.

The generated plasma exits under pressure to the outside via nozzle (3) and aperture (2) and forms a stable plasma jet (9). For the fan-shaped expansion of the jet, the nozzle may have an expansion device in the direction of the jet.

The plasma jet is preferably further stabilized via a swirl stabilization of the working gas. This makes very exact jet geometries possible, e.g., filament-shaped high pressure plasma jets.

A further embodiment of the plasma torch according to the present invention provides devices which magnetohydrodynamically stabilize the highly ionized plasma. Electromagnetic apertures in the outlet area of the plasma jet are provided for this purpose, for example.

In contrast to the known torches for remelting surfaces which use laser energy or arcs, the torch according to the present invention is characterized by a long service life and high operating reliability due to its microwave energy source.

Another embodiment of the plasma torch according to the present invention provides supply devices (31) with which liquid or solid substances may be fed into the plasma jet (9). The supply of solid particles (41) to plasma jet (9) close to the plasma cone on the surface (21) to be remelted is schematically shown in FIG. 2.

A ring nozzle around plasma jet (9) is provided in a further advantageous embodiment of the supply device. Plasma jet (9) and the particle or liquid jet preferably run concentrically to one another. 

1. A method for remelting a metallic surface subjected to thermal-mechanical stress during use, the method comprising: generating a plasma jet using microwave impact on a carrier gas so as to generate a high pressure plasma jet having a pressure higher than atmospheric pressure; applying the plasma jet to the metallic surface in a localized area so as to remelt a surface layer; and allowing the surface layer to solidify, thereby undergoing a structure refinement, the solidified surface layer being structurally refined in comparison with the surface layer before the plasma jet was applied to the metallic surface.
 2. The method as recited in claim 1, wherein the pressure of the plasma jet is from 0.1 MPa to 0.8 MPa.
 3. The method as recited in claim 1, wherein the carrier gas includes at least one of the gases He, Ar, N₂, H₂, O₂, CO₂, H₂O, CH₄ and C₂H₆.
 4. The method as recited in claim 1, wherein the carrier gas is formed by air.
 5. The method as recited in claim 1, wherein the plasma jet has a length greater than 5 cm.
 6. The method as recited in claim 1, wherein the plasma jet is expanded in a fan-shaped manner.
 7. The method as recited in claim 1, further comprising supplying substances to the plasma jet between a nozzle outlet aperture and the metallic surface.
 8. The method as recited in claim 7, wherein the substances are solid substances formed by ceramic powders.
 9. The method as recited in claim 7, wherein the substances are liquid substances formed by metal-organic solutions or metal salt solutions.
 10. The method as recited in claim 7, wherein the substances include at least one of solid and liquid substances and wherein the substances form solid particles in the remelted surface layer, the particles being consisting essentially of at least one of Al₂O₃, AlN, MgO, SiC and Si₃N₄.
 11. The method as recited in claim 1, wherein the plasma jet has a power density from 6 kW/cm² to 20 kW/cm² and wherein the applying includes moving the plasma jet over the surface at a speed of from 2 mm/sec to 4 mm/sec.
 12. The method as recited in claim 1, wherein the plasma jet has a power density from 20 kW/cm² to 60 kW/cm² range and wherein the applying includes moving the plasma jet over the surface at a speed of from 3 mm/sec to 10 mm/sec.
 13. The method as recited in claim 1, wherein the metallic surface is formed by a light metal alloy.
 14. The method as recited in claim 1 wherein the surface refinement enhances a surface hardness, a surface strength or a surface ductility of the surface layer in comparison with the surface hardness, the surface strength or the surface ductility of the surface layer before the plasma jet was applied to the metallic surface. 